To assure adequate quality in paper manufacturing and/or processing, optical checks are required, e.g., in the form of transillumination. On one side of the substrate provision is made for an illumination device, while on the other side thereof provision is made for a camera. To achieve a maximum resolution of the photos the substrate should be completely level while being moved past the camera at a fixed distance therefrom. This is unproblematic when strip-like material is processed. In processing sheet-like substrates having a front and rear edge of the sheet, however, this is difficult. In this context it has to be ensured that the transmitted light checks require a gap in the region of the transport level, allowing a light beam to pass through, and that for this reason the substrate is not guided as it passes across the gap. As a consequence of the absence of such guidance, the substrates passing across the gap, in particular at high transport speeds, may be warped or distorted in the region of the said gap, which would result in imprecise photos, malfunctions or stoppages.
On these premises it is therefore the object of the present invention to improve a device as described initially above in such a way that a high transport speed of the sheet-like substrates is permitted, while a high degree of treatment accuracy and trouble-free operation is ensured.